The Bacillus Calmette-Guérin (BCG) vaccine is primarily used to prevent tuberculosis (TB). Developed in the early 20th century from a strain of Mycobacterium bovis, a bacterium closely related to Mycobacterium tuberculosis, the cause of TB, the BCG vaccine is one of the oldest and most widely used vaccines globally. It is typically administered to infants in countries with a high prevalence of TB. It offers protection against the severe forms of tuberculosis, such as tuberculosis meningitis in children. While its effectiveness against pulmonary tuberculosis in adults is variable and a subject of ongoing research, the BCG vaccine has also shown promise in boosting the immune system to fight other infections. It is being investigated for its potential role in treating bladder cancer. The vaccine is administered via intradermal injection and is known for leaving a small scar at the injection site, a hallmark of BCG vaccination.